Lithographic printing plates (after process) generally consist of ink-receptive areas (image areas) and ink-repelling areas (non-image areas). During printing operation, an ink is preferentially received in the image areas, not in the non-image areas, and then transferred to the surface of a material upon which the image is to be produced. Commonly the ink is transferred to an intermediate material called printing blanket, which in turn transfers the ink to the surface of the material upon which the image is to be produced.
At the present time, lithographic printing plates (processed) are generally prepared from lithographic printing plate precursors (also commonly called lithographic printing plates) comprising a substrate and a photosensitive coating deposited on the substrate, the substrate and the photosensitive coating having opposite surface properties. The photosensitive coating is usually a photosensitive material, which solubilizes or hardens upon exposure to an actinic radiation. In positive-working systems, the exposed areas become more soluble and can be developed to reveal the underneath substrate. In negative-working systems, the exposed areas become hardened and the non-exposed areas can be developed to reveal the underneath substrate. Conventionally, the actinic radiation is from a lamp (usually an ultraviolet lamp) and the image pattern is generally determined by a photomask that is placed between the light source and the plate. Laser sources have been increasingly used to imagewise expose a lithographic printing plate that is sensitized to a corresponding laser. This allows the elimination of the photomask film, reducing material, equipment and labor cost. Among the lasers useful are infrared lasers (about 830 nm or 1064 nm), FD-YAG laser (about 532 nm), violet laser (about 405 nm), and ultraviolet laser (such as about 375 nm).
The exposed plate is usually developed with a liquid developer such as an alkaline aqueous developer to bare the substrate in the non-hardened or solubilized areas. On-press developable lithographic printing plates have been introduced in recent years. Such plates can be directly mounted on press after exposure to develop with ink and/or fountain solution during the initial prints and then to print out regular printed sheets. No separate development process before mounting on press is needed. Among the patents describing on-press developable lithographic printing plates are U.S. Pat. Nos. 5,258,263, 5,516,620, 5,561,029, 5,616,449, 5,677,110, 5,811,220, 6,014,929, 6,071,675, 6,482,571, 6,737,220, 6,994,028, 6,969,575, and 6,949,327.
Photopolymerizable composition based lithographic plates (also called photopolymer plates) are very attractive because of the high durability of photopolymer plates. Examples of such plates are described in U.S. Pat. Nos. 5,607,816, 5,910,395, 6,153,356, 6,232,038, 6,482,571, 6,576,401, 6,881,532, 7,189,494 and 7,291,438, and U.S. Pat. App. Pub. Nos. 2003/0186165, 2004/0229165, 2005/0266349, 2006/0046199, and 2007/0202443. Such plate generally comprises on a substrate a photosensitive layer comprising a polymeric binder, a polymerizable monomer, and an initiator. For plate developed with alkaline developer, an alkaline soluble polymeric binder is generally used in the photosensitive layer, in order to achieve alkaline solubility. For plate developed with non-alkaline developer, an alkaline insoluble polymeric binder is generally used, because alkaline insoluble polymeric binder generally gives better durability (than an alkaline soluble polymeric binder with similar backbone structure). The combination of both alkaline soluble polymeric binder and alkaline insoluble polymeric binder in the photosensitive layer is not expected because of the contradicting properties as mentioned above.
The inventor has found, surprisingly, photopolymer lithographic plates having both alkaline soluble polymeric binder and alkaline insoluble polymeric binder can give excellent combined durability, developability and coatability. Such combined properties are desirable for the manufacture and use of lithographic plates.